Field of the Invention
The present invention described herein generally ultra-low magnetic field object discrimination methods, more particularly, an ultra-low magnetic field object discrimination method for discriminating measurement targets using a measured spin-lattice relaxation time while varying a prepolarization magnetic field or a readout magnetic field with an unsaturated spin-lattice relaxation time.
Background Art
Nuclear magnetic resonance (hereinafter referred to as “NMR”) is a phenomenon that a magnetic spin of the nuclei of atoms precesses in resonance with the magnetic field when a magnetic field is applied to nuclei of atoms constituting all substances. A main magnet of a conventional NMR must spatially uniformly generate a magnetic field of 0.1 Tesla to several Teslas. Therefore, a superconducting main magnet generating the magnetic field is large in volume and high in cost.
Low magnetic field/ultra-low magnetic NMR divides a magnetic field generated by the main magnet of the conventional NMR into a prepolarization magnetic field and a readout magnetic field. Thus, the strength of the readout magnetic field may decrease as tens of microTeslas or several microTeslas.
Low magnetic field/ultra-low magnetic field NMR includes a prepolarization coil generating the prepolarization magnetic field and a readout coil generating the readout magnetic field. The prepolarization coil generates a prepolarization magnetic field to prepolarize a measurement target object. Afterwards, the prepolarization magnetic field is eliminated and the readout magnetic field is applied to the target object, and then a detector measures a nuclear magnetic resonance signal that emerges while the magnetization of the target object is relaxed. Thus, the prepolarization coil has only to generate a strong magnetic field although the uniformity of the prepolarization magnetic field is reduced. In addition, the readout coil has only to generate a uniform but weak readout magnetic field. Therefore, low magnetic field/ultra-low magnetic field NMR may be simple in structure and low in cost. As the strength of the readout magnetic field decreases, a readout signal frequency corresponding to a Larmor frequency in proportion to the strength of a magnetic field decreases as several kilohertz (kHz) or hundreds of Hertz (Hz).
Therefore, it is possible to measure a phenomenon that does not occur in conventional high magnetic field NMR. In the low magnetic field/ultra-low magnetic field NMR, distortion caused by a metal is significantly low. As a result, a material inside a metal can or a packaged material may be non-destructively measured.